1. Field of the Invention
This invention relates to key operated locks and more particularly, but not by way of limitation, to locks forming members of a set operated by keys constructed from identical key blanks.
2. Brief Description of the Prior Art
In many businesses and other organizations in the United States, it has become standard practice to customize locks to the particular organization. Such customization is carried out by configuring the key slots in the locks to accept keys made only from key blanks having a unique pattern of longitudinal grooves formed in the sides of such blanks. The supplier of the locks and keys will provide appropriate key blocks only to persons authorized to receive such blanks by the organization. The purpose of this exclusivity is to increase the difficulty of unauthorized entry into the organization's physical establishment. It is not sufficient for purposes of entry that one have a key having an edge contour appropriate to a particular lock; the key must also have been made from a particular key blank.
In the course of time, the security of an organization using such a lock system may become compromised by the loss of control of one or more keys. For example, an organization may lose control of a key by the loss thereof by an employee or by the failure of an employee to return his keys to the organization when he leaves its employ. Usually, the circumstances of the loss in control of keys are not such that the expense of procuring a complete new set of locks requiring a new key blank design is justified. Under these circumstances, the locks fitting the lost key are changed to require a new edge contour but the design of the key blank is not changed. Locks have been developed in which a change involving only a new edge contour may be carried out rapidly without disassembling the lock. An example of such a lock is disclosed in U.S. Pat. No. 3,589,153, issued June 29, 1971 to Lewis J. Hill.
Although the added security afforded by customized keys is lost in the course of time by loss of keys, changes in key slot and key blank design have been rare. While routine changes of this type are desirable, the cost has generally been prohibitive.
The excessive expense of changing a lock system to require a new key blank design has arisen, in the past, from the manner in which foreign keys have usually been excluded. In general, locks designed for exclusive use by one organization include a plug rotatable within a lock body for unlocking doors, padlocks and the like. The key is inserted into a slot extending longitudinally into the plug and rotated to rotate the plug. Heretofore, locks have been customized for a particular key blank by forming a set of ridges on the portions of the plug forming the walls of the slot, the ridges extending parallel to the longitudinal axis of the slot. The key blanks are provided with a corresponding set of grooves so that the ridges and grooves will mesh when the key is inserted into the slot. If the key is made from a different blank, meshing of one ridge and groove will, in general, prevent meshing of at least one other ridge and groove so that at least one ridge will obstruct a portion of the key to prevent insertion of the key into the key slot.
With exclusivity based on ridges formed in a key slot, it is necessary to change plugs in order to change the in-house key blank design. Since it is not economically feasible to add ridges within the narrow confines of a key slot, the plug cannot be changed to bar insertion of a blank for which it was originally manufactured.
The problem of expense has been exacerbated in the past by the manner in which the ridges in the key slot have been formed. It will be recognized that the more precisely the ridges are positioned and shaped, the more precise must be the match between the ridges and grooves and, accordingly, the more secure the lock system. In a high quality lock, a broaching operation has been used heretofore to establish close tolerances in the positioning and shaping of the ridges. The plug has been initially rough cast from a readily machinable, and relatively expensive, material, and an appropriately shaped broach, ground from tool steel, has been driven into the key slot to provide the final shape of the ridges. The broach adds considerably to the cost of locks, whether the cost be incurred when the locks are initially purchased or when the plugs are changed.
The broaches used in key slot shaping operations are ground from tool steel and may be several feet in length. Clearly, the cost of such broaches is not inconsequential. Moreover, even with proper care, broaches are subject to breakage. Since the costs of broaching must be recovered by the manufacturer in the price he receives for the plugs, the plugs are relatively expensive items.
A second problem is also associated with locks having broached key slots. In general, a considerable time lapse occurs between the recognition that the change in key slot and key blank design must be made and the effectuation of the change. The manufacture of the broach to form the key slots and the subsequent manufacture of the plugs for the locks requires an undesirable time lag in meeting a known compromise of a lock system.
It is known to use a pin in a lock plug to mate with a groove in a key for guiding the key within a key slot. Such a pin, used for this purpose, has been disclosed in U.S. Pat. No. 1,462,869, issued to Primak. It is also known to provide multiple lugs or ridges within a key slot to prevent the insertion of keys other than keys specifically designed for the lock in which the key slot is formed. Wohlmuth, U.S. Pat. No. 471,543, discloses a lock which is opened by insertion of a key into a key slot and which uses lugs or ridges in the key slot to prevent insertion of a foreign key. However, the problem of constructing a lock to permit rapid and inexpensive change of key slot and key blank design has heretofore remained unsolved.